LTE Physical Structure
3GPP (3rd Generation Project Partnership) LTE (Long Term Evolution) supports a type 1 radio frame structure applicable to FDD (Frequency Division Duplex) and a type 2 radio frame structure applicable to TDD (Time Division Duplex).
FIG. 1 shows the structure of a type 1 radio frame. The type 1 radio frame includes ten subframes, and one subframe consists of two slots.
FIG. 2 shows the structure of a type 2 radio frame. The type 2 radio frame includes two half frames, and each half frame is composed of five subframes, a downlink pilot time slot DwPTS, a guard period GP, and an uplink pilot time slot UpPTS. One subframe includes two slots. The DwPTS is used for initial cell search, synchronization or channel estimation in user equipment. The UpPTS is used for channel estimation in a base station and uplink transmission synchronization of the user equipment. The guard period is used to remove interference generated on an uplink due to multi-path delay of a downlink signal between the uplink and downlink. One subframe consists of two slots irrespective of radio frame type.
FIG. 3 shows a slot structure of an LTE downlink. As shown in FIG. 3, a signal transmitted in each slot may be represented by a resource grid composed of NRBDL NscRB subcarriers and NsymbDLOFDM (Orthogonal Frequency Division Multiplexing) symbols. Here, NRBDL represents the number of resource blocks (RBs) on the downlink, NscRB represents the number of subcarriers constructing one RB, and NsymbDL represents the number of OFDM symbols in one downlink slot.
FIG. 4 shows a slot structure of an LTE uplink.
As shown in FIG. 4, a signal transmitted in each slot may be represented by a resource grid composed of NRBUL NSCRB subcarriers and NsymbUL OFDM symbols. Here, NRBUl represents the number of RBs on the uplink, NSCRB represents the number of subcarriers constructing one RB, and NsymbUL represents the number of OFDM symbols in one uplink slot.
A resource element is a resource unit defined by indexes (a, b) in the uplink slot and downlink slot and represents one subcarrier and one OFDM symbol. Here, a is an index in the frequency domain and b is an index in the time domain.
FIG. 5 shows the structure of a downlink subframe. Referring to FIG. 5, a maximum of three OFDM symbols located at the front of the first slot in one subframe correspond to a control region allocated to a control channel. The remaining OFDM symbols correspond to a data region allocated to a physical downlink shared channel (PDSCH). Examples of a downlink control channel used in the 3GPP LTE include PCFICH (Physical Control Format Indicator Channel), PDCCH (Physical Downlink Control Channel), PHICH (Physical Hybrid ARQ Indicator Channel), etc.
Definition of Multi-Antenna (MIMO) Technology
MIMO (Multi-Input Multi-Output) is a method capable of improving transmission and reception data efficiency by using multiple transmission antennas and multiple receiving antennas. That is, MIMO is a technique that increases capacity or improve performance using multiple antennas at a transmitter or a receiver of a wireless communication system. The MIMO is referred to as multi-antenna hereinafter.
A multi-antenna technology is an application of a technique that collects data fragments received through multiple antennas to accomplish a message instead of receiving the message through a single antenna path. The multi-antenna technology is considered to be a next-generation mobile communication technology which can be widely used for mobile communication terminals and relays because the multi-antenna technology can improve a data transfer rate in a specific range or increase a system range for a specific data transfer rate. Furthermore, the multi-antenna technology is attracting attention as a next-generation technology capable of overcoming the limitation of mobile communication transmission capacity that has reached the limit due to extension of data communication.
FIG. 6 shows a configuration of a conventional MIMO communication system. As shown in FIG. 6, when the number of transmission antennas and the number of receiving antennas are simultaneously increased to NT and NR respectively, a channel transmission capacity increases in proportion to the number of antennas in theory, distinguished from a case in which only transmitter or receiver uses multiple antennas. Accordingly, a transmission rate and frequency efficiency can be improved. The transmission rate can be increased by the product of a maximum transmission rate R0 when a single antenna is used and a rate of increase Ri represented by Equation 1 according to increase in the channel transmission capacity theoretically.Ri=min(NT,NR)   [Equation 1]
For example, a MIMO communication system using four transmission antennas and four receiving antennas can acquire a transmission rate four times the transmission rate of a single antenna system in theory. Since the theoretical capacity increase of the multi-antenna system was proved in the mid-90s, various techniques for improving a data transfer rate have been actively studied and some of the techniques are reflected in standards of wireless communications such as 3rd generation mobile communication and next-generation wireless LAN.
MIMO related researches that have been performed so far involve information theory researches related to MIMO communication capacity calculation in various channel environments and multi-access environments, researches on radio channel measurement and modeling, researches on space-time signal processing techniques for improving transmission reliability and transmission rate, etc.
Channel Estimation
In a wireless communication system environment, fading occurs due to multi-path delay. A process of compensating for a signal distortion caused by an abrupt environment variation due to fading so as to restore a transmission signal is referred to as channel estimation. The channel estimation is performed using a signal that both a transmitter and a receiver know, in general. The signal known to both the transmitter and receiver is referred to as a pilot signal or a reference signal (RS).
In a wireless communication system using OFDM transmission method, the reference signal is allocated to all subcarriers or allocated between data subcarriers.
To obtain channel estimation performance gain, a symbol composed of a reference signal only, such as a preamble signal, is used. When the symbol is used, channel estimation performance can be improved, compared to a method of allocating a reference signal between data subcarriers, since the reference signal has a high density generally. In this case, however, data traffic decreases. To increase the data traffic, a method of allocating the reference signal between data subcarriers is used. When this method is used, the density of the reference signal is reduced so as to deteriorate channel estimation performance. Accordingly, appropriate arrangement for minimizing the channel estimation performance deterioration is required.
The receiver performs channel estimation using a reference signal through the following process. The receiver estimates channel information between the receiver and the transmitter from a received signal since the receiver knows information on the reference signal. The receiver can correctly demodulate data transmitted from the transmitter using an estimated channel information value.
When the reference signal transmitted from the transmitter is p, channel information to which the reference signal is subjected while being transmitted is h, thermal noise generated in the receiver is n, and a signal received by the receiver is y, the received signal y can be represented by y=h·p+n. Here, since the receiver knows the reference signal p, channel information ĥ can be estimated using the reference signal p as expressed by Equation 2.ĥ=y/p=h+n/p=h+{circumflex over (n)}  [Equation 2]
Here, the accuracy of the channel estimation value ĥ obtained using the reference signal p is determined based on {circumflex over (n)}. Accodingly, {circumflex over (n)} needs to converge on 0 in order to estimate accurate ĥ, and thus it is necessary to perform channel estimation using a large number of reference signals. If a channel is estimated using a large number of reference signals, the influence of {circumflex over (n)} can be minimized.
User Specific Reference Signal Allocation Method in 3GPP LTE Downlink System
Among the above-described radio frame structures supported b 3GPP LTE, the structure of the radio frame applicable to FDD is described in detail. One frame is transmitted for 10 msec. One frame includes ten subframes. One subframe is transmitted for 1 msec.
One subframe is composed of 14 or 12 OFDM (Orthogonal Frequency Division Multiplexing) symbols, and one OFDM symbol uses 128, 256, 512, 1024, 1536, or 2048 subcarriers.
FIG. 7 shows the structure of a user equipment (UE)-specific downlink reference signal in a subframe using normal cyclic prefix (CP), in which one TTI (Transmission Time Interval) has 14 OFDM symbols. In FIG. 7, R5 represents a UE -specific reference signal and l represents the location of an OFDM symbol on the subframe.
FIG. 8 shows the structure of a UE-specific downlink reference signal in a subframe using extended cyclic prefix (CP), in which one TTI has 12 OFDM symbols.
FIGS. 9, 10 and 11 respectively show structures of UE-common downlink reference signals for systems respectively having 1, 2 and 4 transmission antennas when 1 TTI has 14 OFDM symbols. In FIGS. 9, 10 and 11, R0, R1, R2 and R3 respectively represent pilot symbols for transmission antenna 0, transmission antenna 1, transmission antenna 2 and transmission antenna 3. A subcarrier to which the pilot symbol of each transmission antenna is used doe not carry a signal in order to remove interference of all transmission antennas other than the transmission antenna transmitting the pilot symbol.
The UE-specific downlink reference signals shown in FIGS. 7 and 8 can be used simultaneously with the UE-common downlink reference signals shown in FIGS. 9, 10 and 11. For example, OFDM symbols 0, 1 and 2 of a first slot transmitting control information can use the UE-common downlink reference signals shown in FIGS. 9, 10 and 11 and the remaining OFDM symbols can use the UE-specific downlink reference signals.
Meantime, a downlink reference signal for each cell can be multiplied by a pre-defined sequence (for example, Pseudo-random (PN), m-sequence, etc.) and transmitted so as to reduce signal interference of a pilot symbol received from a neighboring cell at the receiver to thereby improve channel estimation performance. A PN sequence is applied based on OFDM symbols in one subframe. The PN sequence may be applied differently depending on cell ID, subframe number, OFDM symbol location, and user equipment ID.
For example, in the case of the structure of 1Tx pilot symbol of FIG. 9, two pilot symbols of one transmission antenna are used for a specific OFDM symbol including pilot symbols. 3GPP LTE systems include a system having several bandwidths such as 6 RBs (resource blocks) to 110 RBs. Accordingly, the number of pilot symbols of one transmission antenna in one OFDM symbol including pilot symbols is 2×NRB, and the sequence multiplied by the downlink reference signal for each cell needs to have a length of 2×NRB. Here, NRB represents the number of RBs depending on bandwidth and the sequence may use a binary sequence or a complex sequence. The following Equation 3 shows an example of the complex sequence.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                                                          ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB              max                                -          1                                    [                  Equation          ⁢                                          ⁢          3                ]            
Here, NRBmax denotes the number of RBs corresponding to a maximum bandwidth, which may be determined as 110 according to the above explanation, and C denotes a PN sequence which can be defined as Gold sequence of length-31. For the UE-specific downlink reference signals, Equation 3 can be represented as Equation 4.
                                          r            ⁡                          (              m              )                                =                                                    1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                  2                          ⁢                          m                                                )                                                                                            )                                      +                          j              ⁢                                                          ⁢                              1                                  2                                            ⁢                              (                                  1                  -                                      2                    ·                                          c                      ⁡                                              (                                                                              2                            ⁢                            m                                                    +                          1                                                )                                                                                            )                                                    ,                                  ⁢                                  ⁢                  m          =          0                ,        1        ,        …        ⁢                                  ,                              2            ⁢                          N              RB              PDSCH                                -          1                                    [                  Equation          ⁢                                          ⁢          4                ]            
In Equation 4, NRBPDSCH denotes the number of RBs corresponding to downlink data allocated to specific user equipment. Accordingly, a sequence length may depend on the quantity of data allocated to user equipment.
The above-described UE-specific downlink reference signal structures can transmit only one data stream, and cannot transmit a plurality of streams because they cannot be simply extended. Accordingly, the UE-specific downlink reference signal structures need to be extended to transmit a plurality of data streams.
User Equipment Positioning Method
The necessity of user equipment positioning increases for many operations due to various applications in real life. Widely known user equipment positioning methods may be classified into a GPS (Global Positioning System) based method and a terrestrial positioning based method.
The GPS based method detects the position of a user equipment using satellites. The GPS base method requires signals received from at least four satellites and it cannot be used in indoor environments.
The terrestrial positioning based method detects the position of user equipment using a timing difference between signals from base stations. The terrestrial positioning based method requires signals received from at least three base stations. The terrestrial positioning based method can be used in almost all environments although it has position estimation performance lower than that of the GPS based method. The terrestrial positioning based method estimates the position of user equipment using a synchronization signal or a reference signal mostly. The terrestrial positioning based method is defined by the following terms for each standard.
The terrestrial positioning based method is defined as OTDOA (Observed Time Difference Of Arrival) in UTRAN (UMTS Terrestrial Radio Access Network), as E-OTD (Enhanced Observed Time Difference) in GERAN (GSM/EDGE Radio Access Network), and as AFLT (Advanced Forward Link Trilateration) in CDMA 2000.
FIG. 12 illustrates an exemplary downlink OTDOA which is a kind of the terrestrial positioning based method, used in 3GPP standard. Since user equipment generates a reference clock signal on the basis of subframes transmitted from a current serving cell, signals received from neighboring cells have different TDOAs. Here, the TDOA can be measured using a positioning signal of the user equipment, and thus it can be referred to as a RSTD (Reference Signal Time Difference).
FIG. 13 illustrates an example of a user equipment positioning method using OTDOA. The position of the user equipment can be calculated by solving a linear equation using Taylor series expansion (refer to Y. Chan and K. Ho, “A simple and efficient estimator for hyperbolic location,” IEEE Trans. Signal Processing, vol. 42, pp. 1905-1915, August 1994).
The above-mentioned user equipment positioning method can be performed using a common reference signal (CRS) or a primary synchronization signal/secondary synchronization signal (PSS/SSS), however, it is difficult to satisfy requirements of superior performance and operator only using the CRS or PSS/SSS.
Accordingly, it is necessary to introduce a measurement reference signal for LCS (Location Service). Here, the horizontal axis may represent an OFDM symbol index and the vertical axis may represent a frequency index or subcarrier index.
FIGS. 14 and 15 show structures of subframes including RSs for LCS for OTDOA. FIG. 14 shows a normal CP case and FIG. 15 shows an extended CP case. In FIGS. 14 and 15, an E-IPDL (Evolved-Idle Period Downlink) RS corresponds to an RS for LCS. The RS for LCS may be referred to as a PRS (Positioning Reference Signal).
In FIGS. 14 and 15, the horizontal axis may represent an OFDM symbol index and the vertical axis may represent a frequency index or subcarrier index. As shown in FIGS. 14 and 15, E-IPDL RSs have a diagonal matrix form in one cell. The E-IPDL RSs are evenly distributed in one subframe. That is, if E-IPDL RS elements are combined in one subframe, all of the E-IPDL RSs are transmitted in the entire resource elements. Here, the E-IPDL RSs may be transmitted in a specific resource unit (frequency X symbol) only, or uniformly transmitted over the overall band.
In another cell, E-IPDL RSs may be circularly shifted one by one on the frequency axis and transmitted. In this case, if E-IPDL RSs transmitted by two cells are perfectly synchronized with each other and received, the position of user equipment can be measured without collision between the cells. That is, E-IPDL RS patterns for the cells are configured differently such that the position of the user equipment can be measured without collision of RSs between the cells. Here, collision means a case in which the same RS signal patterns are located on the same time and frequency resources on subframes transmitted from two cells and interfere each other.
The above-mentioned user equipment positioning method can be performed using a synchronization signal or a CRS. A user equipment position estimation error is proportional to the bandwidth occupied by a transmitted synchronization signal or reference signal. In other words, timing resolution increases as bandwidth increases, in general. Accordingly, RS measurement is performed through the following two steps.
(1) First step of performing symbol timing acquisition through a synchronization signal
(2) Second step of time resolution through an RS
However, if the user equipment is located very close to a serving cell, the user equipment may not recognize a signal of the neighbor cell because the signal of the neighbor cell becomes less than the granularity of quantization of an analog-to-digital converter (ADC) due to strong power of the serving cell. That is, a hearibility problem may be generated.
To solve this problem, UTRA standard provides IPDL (Idle Period Downlink) technique that interrupts transmission of all channels of the serving cell. Generally, the frequency of an idle period is one slot (approximately 667 μs) per 100 ms (that is, approximately 0.7%). During the idle period, the user equipment can receive a pilot signal of a neighbor cell even if a signal with high intensity is received from the serving cell in the same frequency band. Furthermore, a signal of the serving cell can be measured more accurately through an idle period of a first best neighbor cell signal.
Even in this case, however, the conventional synchronization signal and CRS(Common Reference Signal)/DRS(Dedicated Reference Signal) need to be transmitted for other user equipments, and thus user equipment positioning performance may be deteriorated when these signals are transmitted.
Moreover, in the structures of FIGS. 14 and 15, LCS RSs transmitted from multiple cells may be received without being synchronized.
FIG. 16 illustrates a state that LCS RSs transmitted from multiple cells are received without being synchronized. When the LCSs RSs are received without being synchronized, as shown in FIG. 16, the possibility of collision of RSs of all cells increases in the case of a diagonal structure. On the assumption that FIG. 16 corresponds to a normal CP case, if cells A and B transmit different RS patterns and user equipment receives the RS patterns with an offset corresponding to one OFDM symbol, collision between RSs may occur. In this case, collision occurs in every RE, and thus measurement performance is deteriorated even if different sequences are used.
This problem is not limited to the LCS RS only and may be generated in the normal RS and CoMP (Coordinated Multi-Point) RS.